Field of the Invention
The present invention relates to an optical fiber.
Description of the Related Art
An optical fiber has been installed even on the inside of buildings, houses, or the like according to the spread of Fiber To The Home (FTTH). According to this, an optical fiber has attracted attention in which an excessive loss, so-called bending loss (a macro bend loss) which occurs at the time of imparting bending properties, is reduced.
By using an optical fiber having a low bending loss, prevention of instantaneous interruption of a signal due to the loss which occurs at the time of bending the optical fiber, a reduction in installation costs due to handling simplification, and the like have been expected.
ITU-T Recommendation G.657 is used as the standard of an optical fiber in which the bending loss is reduced compared to a standard single mode optical fiber (SSMF) while being based on ITU-T Recommendation G.652 which is the standard of the standard single mode optical fiber (SSMF).
For example, the following methods have been proposed as a method of improving the bending loss.
(1) Increasing Refractive Index of Core
By increasing the refractive index of the core, a mode field diameter (MFD) decreases compared to S-SMF, and thus, containment of light with respect to the core is improved. Therefore, the bending loss of the optical fiber can be reduced. In this case, in order to match dispersion to G.652, it is preferable that so-called depressed refractive index profile in which the refractive index of the cladding near the core is reduced be adopted (for example, refer to Published Japanese Translation No. 2013-520711 of the PCT International. Publication and K. Okamoto and T. Okoshi, “Computer-aided synthesis of the optimum refractive index profile for a multimode fiber,” IEEE Trans. Microwave Theory Tech., vol. MTT-25, pp. 213-221, 1976). In addition, Published Japanese Translation No. 2013-520711 of the PCT International Publication shows that by adjusting the refractive index of an internal-layer cladding region, it is possible to maintain a balance between both of the characteristics, that is, the MFD and the bending loss.
A product corresponding to a bending radius up to 15 mm which is based on G.657.A1 is used as such a type of optical fiber.
(2) Adopting Refractive Index Profile having Trench-Shape
By adopting the trench-shape refractive index profile having a low refractive index portion at a location away from the core of the cladding, containment of the light with respect to the core can be improved and the bending loss can be reduced (for example, refer to Japanese Unexamined Patent Application, First Publication No. S63-43107).
A product corresponding to a bending radius up to 10 mm which is based on G.657.A2 or G.657.B2 and a product corresponding to a smaller bending radius up to 7.5 mm which is based on G.657.B3 are used as such a type of optical fiber. In addition, a product corresponding to a bending radius up to 7.5 mm which is based on G.657.B3 and of which other optical properties are based on the standard of G.657.A series are used as such a type of optical fiber.
(3) Forming Hole to Cladding
An optical fiber (for example, a hole-assisted fiber (HAF)) in which a hole along a longitudinal direction of the optical fiber is formed at a location away from the core of the cladding or an optical fiber in which a fine structure including a plurality of independent openings (for example, ClearCurve® manufactured by Corning Inc.) can improve containment of light with respect to the core. Therefore, the bending loss can be reduced (for example, refer to PCT International Publication No. WO2004/092793 and Published Japanese Translation No. 2009-543126 of the PCT International Publication).
A product corresponding to a bending radius up to 7.5 mm which is based on G.657.B3 is used as such a type of optical fiber.
In the above-described method, an optical fiber having a high refractive index of the core can have a lower bending loss. However, according to this, the mode field diameter decreases, and a connection loss with respect to SSMF increases. For this reason, there is a limit for reducing the bending loss.
In an optical fiber having a trench type refractive index profile, it is necessary to form the plurality of layers having a different refractive index, and thus, a manufacturing step of a preform becomes complicated.
In an optical fiber having a hole, a manufacturing step becomes complicated compared to an optical fiber having a solid structure. In addition, an advanced drawing technology is required, and thus, the manufacturing is not easily performed.
The present invention takes into consideration of the above circumstances and provides an optical fiber in which the bending loss can be reduced without increasing the connection loss and which is easy to manufacture.